1. Field of the invention
The present invention relates to an apparatus for reproducing recorded data, and more particularly to an apparatus for reproducing data recorded on a disc type recording medium in the so-called sampled format.
2. Description of the Background Information
A disc which is written to once, such as the so-called DRAW (Direct Read After Write) disc carries, for example, thirty-two (32) sectors per one track. Each sector is made up of forty-three (43) servo blocks, and each servo block includes a servo area of two bytes and a data area of sixteen (16) bytes following the servo area. In each servo area two wobbled pits and one clock pit are previously formed as illustrated in FIG. 1. The wobbled pits are disposed on left and right sides of the track center, serving as a marker for the tracking servo operation. The position of the leading one of the wobbled pits is tangentially displaced from each other per sixteen (16) tracks. When the information detecting point of the pickup (a light spot for detecting information) moves on the track center, the decreases in quantity of light by the reflection at the left and right wobbled pits becomes equal, while decreases in quantity of the light at the left and right wobbled pits differs depending upon the direction and magnitude in deviation of the detecting point from the track center. Therefore, a tracking error signal can be generated from the difference between the decreased amounts (the difference between levels of RF signals) at two positions and this tracking error signal is held during the period of the succeeding data area.
As mentioned above, the distance between two adjacent wobbled pits varies at intervals of 16 tracks. By sensing the changes of the distance, it is possible even in a high-speed search mode to correctly count the number of tracks (this operation being designated 16-track counting).
Furthermore, the distance D between the trailing one of the two wobbled pits and a clock bit is set to a particular distance which does not appear in the data area. Therefore, the distance D can be detected as a synchronizing signal. Various timing signals are generated on the basis of the detected synchronizing signal. A clock signal is generated on the basis of a detection signal corresponding to the clock pits. The mirror portion between the pits extending through the distance D is used as a focus area during which a focus error signal is produced and the focus error signal is held during the period of the succeeding data area.
When a DRAW disc having a diameter of 5 inches with servo pits previously recorded thereon, for example, is rotated at 1800 rpm, the clock pulse signal generated in the RF signal due to the existence of the clock pits will have a repetition frequency of 41.28KHz.
Japanese Patent Application Laid-open No. P63-53760 specifically discloses an example of a recording and reproducing system which is arranged to record the data in the data areas, and to reproduce the recorded data while reading the address data and information data previously recorded on the DRAW disc following the servo area. This system is known as servo signal section.
When, in such a system, data is recorded on the DRAW disc, changes occur at the recording position of the data with respect to the servo area. This occurs at the prepit (hereinbefore referred to as the servo area) section of the disc because of variations in the sensibility of the material of the disc, and variations of the recording light power. When the recording position of data changes against the prepit section, the relation of the timing position between the clock pulse and the data section in the reproduced RF signal read from the disc is different from the time of recording. Thus, the error rate of the data becomes worse and the recorded data may not be stably reproduced.